I installed a tranny cooler a couple of years back. There are several sizes (capacities). What is going on is that you are adding a small radiator that mounts (typically, and in my case) onto the front of the radiator (in my case the lower drivers side). The kit was about $60. The tranny fluid lines that goes to the existing cooler in the base of the radiator are tapped into and the new cooling unit is connected "in series". There is a preferred flow of the oil into either the new cooler first or second...the instructions will guide you.

My unit is about 5 X 8 inches. I do not have a tranny oil temp gauge but do get a good feel (pun intended) for whats going on just by touching the shift lever. You can tell warm from hot.